The present invention relates to an optical functional film and more particularly to an optical functional film suitable for use as an antireflection film in various displays of word processors, computers, and television, surfaces of polarizing plates used in liquid crystal displays, optical lenses, such as sunglass lenses of transparent plastics, lenses of eyeglasses, finder lenses for cameras, covers for various instruments, and surfaces of window glasses of automobiles and electric railcars, and a process for producing the same.
Transparent substrates, such as glasses and plastics, are used in curve mirrors, back mirrors, goggles, window glasses, displays of personal computers and word processors, and other various commercial displays. When visual information, such as objects, letters, and figures, is observed through these transparent substrates or, in the case of mirrors, when an image from a reflecting layer is observed through the transparent substrates, light reflects at the surface of the transparent substrates, making it difficult to see the visual information through the transparent substrates.
Conventional methods for antireflection of light include, for example, a method wherein an antireflection coating is coated on the surface of glass or plastics, a method wherein a very thin film of MgF.sub.2 or the like having a thickness of about 0.1 .mu.m or a metal deposited film is provided on the surface of a transparent substrate, such as glass, a method wherein an ionizing radiation curing resin is coated on the surface of plastics, such as plastic lenses, and a film of SiO.sub.2 or MgF.sub.2 is formed thereon by vapor deposition, and a method wherein a coating having a low refractive index is formed on a cured film of an ionizing radiation curing resin.
An about 0.1 .mu.m thin film of MgF.sub.2 formed on the above glass will now be described in more detail. It is already known that, when incident light perpendicularly enters a thin film, in order for the antireflection film to prevent the reflection of light by 100% and to pass light by 100% therethrough, relationships represented by the equations (1) and (2) should be met (see "Science Library" Physics=9 "Optics," pp.70-72, 1980, Science Sha Ltd., Japan). EQU n.sub.o =.sqroot.n.sub.g Equation (1) EQU n.sub.o h=.lambda..sub.o /4 Equation (2)
wherein .lambda..sub.o represents a particular wavelength, n.sub.o represents the refractive index of the antireflection film at this wavelength, h represents the thickness of the antireflection film, and n.sub.g represents the refractive index of the substrate.
It is already known that the refractive index n.sub.g of glass is about 1.5, the refractive index n.sub.o of an MgF.sub.2 film is 1.38 and the wavelength .lambda..sub.o of incident light is 5500 .ANG. (reference). When these values are substituted in the equation (2), the results of calculation show that the thickness h of the antireflection film is about 0.1 .mu.m in terms of the optimal thickness.
From the equation (1), it is apparent that prevention of the reflection of light by 100% can be attained by the selection of such a material that the refractive index of the upper coating is approximately equal to a value of square root of the refractive index of the lower coating. The antireflection of light by utilizing the above principle, i.e., by making the refractive index of the upper coating slightly lower than the refractive index of the lower coating, has hitherto been carried out in the art.
Further, the surface of displays has hitherto been subjected to glare shielding treatment so that the reflection of light from the exterior or interior of displays could be diffused by the surface of the displays to shield glare. The glare shielding treatment has been carried out, for example, by a method wherein a resin containing a filler, such as silicon dioxide, is coated on the surface of a display, or a method wherein an antiglare substrate with a resin containing a filler, such as silicon dioxide, being coated thereon is applied onto the surface of a display.
In particular, a filmy polarizing element, which serves as an optical shutter, is provided on the surface of displays, such as liquid crystal displays. Since, however, the polarizing element per se has poor hardness, it is protected by a transparent protective substrate, such as glass, a transparent plastic sheet, or a transparent plastic film, to form a polarizing plate. However, the transparent protective substrate of a plastic, such as a transparent plastic sheet or a transparent plastic film, is also likely to be scratched. In order to solve this problem, in recent years, a polarizing plate with a hard property being imparted to the surface thereof has been developed. For example, Japanese Patent Laid-Open No. 105738/1989 describes such a technique.
This publication discloses a transparent protective substrate having excellent hardness and an antiglare property, that is, a triacetate film for light control, which is laminated to a polarizing element to constitute a polarizing plate. Since this triacetate film is formed by providing a cured coating of an ultraviolet curing epoxy acrylate resin on one surface of an unsaponified triacetate film, it has an excellent hard property.
In order to further impart an antiglare property to the above triacetate film having excellent hardness, a resin composition comprising the above ultraviolet curing epoxy acrylate resin and, added thereto, amorphous silica is coated on the surface of a triacetate film followed by curing. In laminating the coated triacetate film onto a polarizing element to form a polarizing plate, the coated triacetate film is first saponified with an alkali for the purpose of enhancing the adhesion to a polarizing element and, at the same time, antistatic purposes and then laminated to a polarizing element to form a polarizing plate.
However, it is noted that, when a layer for imparting a light antireflection property and, at the same time, an antiglare property is provided on a substrate film to form an antiglare-antireflection film, at least layers having these functions and other various layers, such as an adhesive layer, are provided, necessitating the provision of at least one layer, for example, between a substrate film and the outermost layer provided on the substrate film. In this case, the reflection of light occurs in the interface of layers, particularly in the interface of a layer having a relatively large thickness of not less than 0.5 .mu.m such as formed by coating, i.e., a larger thickness than the wavelength of light, deteriorating the antireflection effect of the antireflection film.
On the other hand, for the conventional antireflection film with an antireflection layer being formed on the outermost surface of a transparent substrate film, since the thickness of the antireflection layer is as small as about 0.1 .mu.m, the antireflection film has poor hardness and, at the same time, is likely to be scratched.
Further, for the film having an optical function, such as an antireflection film, optical functional membranes are usually laminated thereon. These optical functional membranes have an unsatisfactory gas barrier property and, hence, have a poor moistureproofness. In particular, a polarizing element used in a liquid crystal display has poor moistureproofness, and, therefore, moistureproofness should be imparted thereto.
Accordingly, the first object of the present invention is to provide, with respect to optical functional materials constituting optical materials, such as an antireflection film and an antireflection film, an optical functional film having excellent gas barrier properties, such as moistureproofness.
The second object of the present invention is to provide an antiglare-antireflection film having an antiglare property and/or an antireflection property and, at the same time, capable of reducing the reflection of light in the interface of layers in the interior of the film and a process for producing the same.
The third object of the present invention is to provide an antireflection film having a hard property in addition to properties involved in the second object and a process for producing the same.